1. Field of the Invention
The present invention relates to a display device, and more particularly, to a liquid crystal display (LCD) device with improved image quality and driving method thereof.
2. Discussion of the Related Art
Flat display devices of lightweight and small volume are under development in order to replace cathode ray tubes (CRT). The flat display devices are classified into LCD devices, field emission displays (FED), plasma display panels (PDP), and electro-luminescence (EL) displays. The flat display devices display images on a display panel in accordance with video signals received from the outside.
Driving methods of the flat display devices are classified into the progressive type and the interlace type. The progressive-type driving method displays images on a frame basis, i.e., by a unit of image signals for one screen. Representative display devices using the progressive-type driving method are computer monitors, PDP, and LCD devices. For example, computer monitors uses image signals provided in the form of the progressive type. That is, the image signals of computer monitors are supplied on a frame basis. In the interlace-type driving method, image signals for one screen, i.e., one frame, are divided into image signals for an odd field and image signals for an even field and the image signals are displayed in order of the odd field and the even field. The representative display devices using this interlace-type driving method are televisions (TV). Televisions use image signals provided in the form of the interlace type. That is, image signals of one frame in TVs are divided into image signals for an odd field and image signals for an even field and supplied on a field basis.
An LCD device has a liquid crystal (LC) panel including pixels arranged in a matrix configuration for displaying images and a drive unit for driving the LC panel. The LC panel has horizontal lines and vertical lines, the pixels defined by the horizontal lines and the vertical lines, and pixel electrodes formed in the pixels. Also, red, green, and blue color filters are formed on the regions that correspond to the pixels. The drive unit includes a gate driver for sequentially supplying scan signals to the horizontal lines, a data driver for supplying predetermined image signals to the vertical lines, and a timing controller for generating control signals for controlling the gate driver and the data driver.
The horizontal lines are sequentially driven by the scan signals supplied from the gate driver and image signals supplied from the data driver are applied to the pixels via the vertical lines to display a predetermined image through the color filters. That is, the image signals for one frame are displayed in response to the sequentially driven horizontal lines. Therefore, the progressive-type driving method is appropriate for the LCD device having the horizontal lines sequentially operating. In other words, because the horizontal lines operate regardless of odd and even horizontal lines, the progressive type is appropriate for the LCD device.
Because image signals of the interlace type are supplied to an LCD-TV, the image signals cannot be properly displayed on the LCD-TV. To solve this problem, the image signals of the interlace type supplied to the LCD-TV are converted into image signals of the progressive type and the converted image signals are displayed on the LCD-TV. However, in such a case, because various devices (e.g., data converter, frame memory) for converting the image signals of the interlace type into the image signals of the progressive type are additionally required for the LCD-TV, the circuits of the LCD-TV becomes complicated and the manufacturing cost increases.
To solve this problem, a method of directly displaying image signals of the interlace type (interlace image signals) on an LCD device has been suggested without converting the interlace image signals into progressive image signals.
As described above, the interlace image signals are divided into the image signals for the odd fields and the image signals for the even fields and supplied to the LCD device. The odd fields have actual pixel data only on the odd horizontal lines and do not have actual pixel data on the even horizontal lines. In contrast, the even fields do not have actual pixel data on the odd horizontal lines but have actual pixel data only on the even horizontal lines. Therefore, the combination of the odd and even fields constitutes one complete frame.
When actual pixel data for odd fields are supplied, the LCD device generates dummy pixel data on the even horizontal lines on the basis of the actual pixel data existing on adjacent odd horizontal lines. Also, when actual pixel data for even fields are supplied, the LCD device generates dummy pixel data on the odd horizontal lines on the basis of the actual pixel data existing on adjacent even horizontal lines.
There are various methods of generating the dummy pixel data. The dummy pixel data are at least smaller than the actual pixel data. Because the actual pixel data exist on the odd horizontal lines of the odd fields and the dummy pixel data also exist on the even horizontal lines of the odd fields, each of the odd fields can constitute one complete frame. Also, because the dummy pixel data exist on the odd horizontal lines of the even fields and the actual pixel data exist on the even horizontal lines of the even fields, each of the even fields can constitute one complete frame.
The LCD device displays the first frame including the odd field by sequentially driving the respective gate lines and displays the second frame including the even field by driving the respective gate lines. Therefore, the LCD device can directly display the interlace image signals.
FIG. 1A is a schematic view illustrating interlace pixel data (interlace image signals) for an odd field supplied to an LC panel according to the related art and FIG. 1B is a schematic view illustrating interlace pixel data for an even field supplied to an LC panel according to the related art.
During the odd field period, actual pixel data are displayed on the odd horizontal lines and dummy pixel data are displayed on the even horizontal lines, as illustrated in FIG. 1A. In contrast, during the even field period, dummy pixel data are displayed on the odd horizontal lines and actual pixel data are displayed on the even horizontal lines, as illustrated in FIG. 1B. As described above, the dummy pixel data can be generated using the actual pixel data on adjacent horizontal lines.
Referring to FIG. 2, in order to improve the image quality of an LCD device, the polarity of the interlace image signals is generally inverted every field and the polarities of the image signals in one field are also configured in a dot-inversion method.
During the odd field (OF) period, actual pixel data having a positive polarity with respect to a common voltage Vcom are charged in predetermined pixels on the odd horizontal lines, while during the even field (EF) dummy pixel data having a negative polarity are charged in the pixels on the even horizontal lines, as illustrated in FIG. 3. The same pixel data may be charged in the respective pixels during the next OF and EF periods. As described above, the absolute values of the actual pixel data are much greater than the absolute values of the dummy pixel data. Accordingly, as the OF and EF periods are repeated, the average voltage (DC voltage) charged in the pixels has a positive polarity with respect to the common voltage Vcom, thereby generating a flicker and an afterimage defect.
To solve these problems, the polarity of the pixel data is inverted every other field (by a unit of two fields), as illustrated in FIG. 4. In detail, actual pixel data having a positive polarity with respect to the common voltage Vcom are charged in predetermined pixels on the odd horizontal lines during the first OF period and dummy pixel data having a positive polarity are charged in the pixels during the first EF period, as illustrated in FIG. 5. During the second OF period, actual pixel data having a negative polarity are charged in predetermined pixels on the odd horizontal lines and dummy pixel data having a negative polarity are charged in the pixels during the second EF period. Such a driving method is applicable to the next OF and EF periods in the same manner described above.
In the driving method described above, the positive actual pixel data charged during the first OF period and the positive dummy pixel data charged during the first EF period cancel out the polarity effects of the negative actual pixel data charged during the second OF period and the negative dummy pixel data charged during the second EF period. Accordingly, the average voltage (DC voltage) becomes almost zero, thereby preventing or minimizing the afterimage defect.
However, although the afterimage defect is prevented or minimized by inverting the polarity of the image signals every other field (by a unit of two fields), a flicker may occur. Referring to FIG. 6, actual pixel data having a positive polarity are charged in predetermined pixels on the horizontal lines during the OF period, dummy pixel data having a positive polarity are subsequently charged in the pixels during the EF period. In that case, because the pixels are charged with the same positive polarity during the OF and EF periods, not all of the actual pixel data charged during the OF period are discharged and some DC voltage remains. Therefore, the remaining DC voltage during the OF period is added to the dummy pixel data, so that dummy pixel data greater than the dummy pixel data are charged during the EF period. Such a process repeatedly occurs every EF period.
Accordingly, actual images are not displayed during the EF periods due to the influence of the remaining DC voltage, thereby generating a flicker. Such a flicker is particularly conspicuous, when pixel data having the same brightness are displayed. For example, when the pixel data of the first OF and the pixel data of the first EF have the same white value, the white color of the first OF is different from the white color of the first EF and a conspicuous flicker occurs due to the remaining DC voltage existing on the horizontal lines of the first OF.